The present invention relates to an apparatus and method for measuring a process variable. More particularly, the present invention relates to an improved method for providing an accurate indication of the location of an interface between a first medium and a second medium in a vessel using time-of-flight of signal reflections and automatically updated reference signals.
The process and storage industries have long used various types of equipment to measure process parameters such as level, flow, temperature, etc. A number of different techniques (such as mechanical, capacitance, ultrasonic, hydrostatic, etc.) provide measurement solutions for many applications. However, many other applications remain for which no available technology can provide a solution, or which cannot provide such a solution at a reasonable cost. For many applications that could benefit from a level measurement system, currently available level measurement systems are too expensive.
In certain applications, such as high volume petroleum storage, the value of the measured materials is high enough to justify high cost level measurement systems which are required for the extreme accuracy needed. Such expensive measurement systems can include a servo tank gauging system or a frequency modulated continuous wave radar system.
Further, there are many applications that exist where the need to measure level of the product is high in order to maintain product quality, conserve resources, improve safety, etc. However, lower cost measurement systems are needed in order to allow a plant to instrument its measurements fully.
There are certain process measurement applications that demand other than conventional measurement approaches. For example, applications demanding high temperature and high pressure capabilities during level measurements must typically rely on capacitance measurement. However, conventional capacitance measurement systems are vulnerable to errors induced by changing material characteristics. Further, the inherent nature of capacitance measurement techniques prevents the use of such capacitance level measurement techniques in vessels containing more than one fluid layer.
Ultrasonic time-of-flight technology has reduced concerns regarding level indications changing as material characteristics change. However, ultrasonic level measurement sensors cannot work under high temperatures, high pressures, or in vacuums. In addition, such ultrasonic sensors have a low tolerance for acoustic noise.
One technological approach to solving these problems is the use of guided wave pulses. These pulses are transmitted down a dual probe transmission line into the stored material, and are reflected from probe impedance changes which correlate with the fluid level. Process electronics then convert the time-of-flight signals into a meaningful fluid level reading. Conventional guided wave pulse techniques are very expensive due to the nature of equipment needed to produce high-quality, short pulses and to measure the time-of-flight for such short time events. Further, such probes are not a simple construction and are expensive to produce compared to simple capacitance level probes.
Recent developments by the National Laboratory System now make it possible to generate fast, low power pulses, and to time their return with very inexpensive circuits. See, for example, U.S. Pat. Nos. 5,345,471 and 5,361,070. However, this new technology alone will not permit proliferation of level measurement technology into process and storage measurement applications. The pulses generated by this new technology are broadband, and also are not square wave pulses. In addition, the generated pulses have a very low power level. Such pulses are at a frequency of 100 MHz or higher, and have an average power level of about 1 nW or lower. These factors present new problems that must be overcome to transmit the pulses down a probe and back and to process and interpret the returned pulses.
The reflected pulses include reflections due to factors which are unrelated to the level of material in the vessel, such as mounting conditions, structures within the vessel, and other environmental factors. Reference signals are used to map the factors that are unrelated to material level so that during level measurement the reflections due to these factors do not interfere with the detection of the reflection on the reflected pulse that is due to the material level in the vessel. The reference signals are accurate when they are collected, however over time there are changes in the factors which are unrelated to material level and this causes the reflections due to these factors to change. If the reference signal is not updated, these changes in the reflections due to the factors unrelated to material level in the vessel can eventually be misinterpreted as level reflections. This causes erroneous output results of the process variable.
Accordingly, a need exists for a method of automatically updating the reference signal on a periodic basis to track the reflections due to factors which are unrelated to the material level in the vessel. Thereby allowing the detection of the reflection due to the material level and the accurate reporting of the appropriate process variable.